1. Field of the Invention
The invention relates to the field of circuits and microsystems comprising one or more piezoelectric resonators. More particularly, the invention relates to the production of such a resonator in integrated form, being produced using a technology generally employed for the production of integrated circuits.
2. Description of the Related Art
Resonators in integrated circuits can be used as filters in mobile telephony applications. A noisy signal is applied to the upper electrode. A filtered signal centered on the resonant frequency of the resonator is recovered on the lower electrode. This frequency is directly determined by the thickness of a piezoelectric film placed between the two electrodes. An assembly of resonators allows a filter function to be constituted. The resonator is used as a filter element of high quality factor.
The resonators are integral with the integrated circuit, all of them having to be acoustically isolated relative to the latter. For this purpose, a support capable of producing such isolation may be provided. The support may comprise an alternation of layers of high acoustic impedance and layers of low acoustic impedance. The support may include a suspended membrane.
The actual resonator comprises an active element, formed from a piezoelectric material, placed between two electrodes. The resonant frequency of the resonator depends essentially on the thickness of the piezoelectric material, on its mechanical properties and on the mechanical properties of the electrodes. Resonators may be connected together in various configurations in order to produce filters. The central frequency of such assemblies depends on the resonant frequency of each of the resonators of which it is composed.
Document U.S. Pat. No. 5,821,833 proposes a device consisting of a stacked-crystal filter, a support substrate and an acoustic reflector placed between the substrate and the filter in order to eliminate undesirable resonant frequencies, namely the principal mode and the third harmonic.
The article “Bulk acoustic wave coupled resonator filters” by K. M. Lakin on pages 8 to 14 of 2002 IEEE International Frequency Control Symposium and PDA Exhibition has reviewed stacked-crystal filters and coupled-resonator filters, in which two resonators are coupled by coupling layers placed between them. The filters of both types are mounted on bulk isolation reflectors supported by a bulk substrate.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.